Mumbai: The Maharashtra Anti-Terrorism Squad (ATS), which has successfully cracked down on terror elements in recent months, has received an anonymous letter threatening murder of its officers in Pune and Mumbai.

"We received an anonymous letter yesterday threatening to kill our officers in Mumbai and Pune. "Inquiry into the matter is on and security has been beefed up around the houses of the officers named in the letter," said a senior ATS officer.

Against the backdrop of the ATS's actions in such cases, some of which had an Islamic State angle, there was speculation whether the letter was sent by the dreaded terror outfit although the ATS officer rejected that possibility.

In the past couple of months, ATS, particularly its Pune unit, has successfully pursued several terror cases. Recently, the Pune unit had found out about a 16-year-old Muslim girl who had been reportedly radicalised by her Islamic State (IS) contacts abroad and brainwashed into heading for Syria.

Wajid Shaikh, one of the four missing youths from Malwani, a suburb of Mumbai, suspected to have joined IS, was also tracked down by the Pune unit of ATS.

The ATS has also registered a case against Ayaz Sultan, another youth from Malwani area of the city, who has been missing since October 30.